This invention relates to a medical diagnostic ultrasound transducer. In particular, a capacitive microelectromechanical ultrasonic transducer and method for using the transducer are provided.
Capacitive microelectromechanical ultrasonic transducer (CMUTs) comprise transducer arrays of a single layer of chambers and associated membranes etched within a silicon wafer. CMUTs provide ultra-wideband phased arrays, and may allow integrated circuit components to be etched on the same wafer as the transducer. Each CMUT element is a hollowed chamber with a membrane subject to externally induced mechanical collapse. The chamber allows the membrane to vibrate, transferring acoustic energy away from the CMUT or converting acoustic energy into electrical signals. Each CMUT or chamber is formed using directionally selective wet or dry etching techniques.
CMUTs are inefficient as compared with conventional piezoelectric devices. For example, a typical CMUT device with a DC bias of 230 volts provides a maximum output pressure of around 33,000 Pascals per volt (P/V). In comparison, an Acuson L5 piezoelectric transducer element outputs pressure of around 46,000 P/V for transmit. Similar relative receive efficiencies are expected. More efficient devices allow lower voltage levels, reducing the complexity of transmit circuitry. In the receive mode, improved efficiency provides better signal to noise ratios, allowing improved image quality at deeper depths.
CMUT devices also have poor mechanical strength. The CMUT devices may break or become inoperable when placed in contact with tissue. The pressure applied from the tissue may collapse or adversely affect the performance of the membrane within the chamber.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below include a CMUT transducer array and associated method for using the CMUT transducer array with improved efficiency and durability. Efficiency is provided by stacking CMUTs in the range dimension (i.e. away from the face of the transducer). A plurality of chambers and associated membranes are stacked along a range dimension or parallel to the direction of acoustic radiation. Because the CMUT transducer element is stacked, ultrasound is transmitted through the plurality of chambers, amplifying the response of the transducer element.
Durability is increased within the transducer by filling the chamber with a nongaseous filler. A liquid, polymer, solid or gas fills the chamber or chambers. The nongaseous filler allows movement of the membrane for transducing between acoustic and electrical energies, but prevents collapse or bottoming out of the membrane.
Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments.